Distributed feedback lasers

ABSTRACT

Improved single-modedness is achieved in distributed feedback (DFB) lasers by minimizing the feedback provided at or near the ends of the DFB grating. The feedback may be minimized at one or both ends of the grating. Feedback is controlled by reducing the coupling to the grating. Coupling is reduced by reducing the depth of the grating&#39;s teeth and/or by increasing the spacing of adjacent teeth.

FIELD OF THE INVENTION

The present invention relates to distributed feedback (DFB) lasers andin particular but not exclusively to DFB semiconductor injection lasers.

BACKGROUND OF THE INVENTION

DFB lasers do not utilise the cavity mirrors of conventional Fabry-Perotsystems, but rather rely on backward Bragg scattering from periodicperturbations of the refractive index and/or gain of the laser medium toprovide the optical feedback necessary for laser operation. DFBstructures have the advantage of providing better frequency stability ofthe mode of oscillation than Fabry-Perot structures.

In semiconductor DFB lasers, the periodic perturbations are generallyprovided by means of a grating formed, usually, in a semiconductor layeradjacent the device's active layer, the teeth and grooves of the gratingextending orthogonally to the device's optical axis. Typically thegrating extends throughout the entire length of the laser, but in somedevices the grating is shorter than the device, the grating ends beingremote from the device ends.

The wavelength sensitivity of the Bragg effect results in DFB lasersexhibiting a high degree of spectral selection. The narrow linewidth ofDFB lasers means that they are very attractive for use in opticalcommunications systems, not least because of the effective increase inbandwidth that can be achieved as a result of the reduced dispersionconsequent on the use of a narrower linewidth source. Unfortunately,however, the optical output of DFB lasers, while of narrow linewidth, isnot absolutely monochromatic: generally two and sometimes threelongitudinal modes will be supported simultaneously, the dominant modehaving the highest intensity output. If the power difference between thedominant and subordinate modes is great enough, the laser's output canbe considered for some purposes to be single mode. The problem lies inensuring a sufficiently great power difference. Typically, the aim is toachieve side to main mode power ratios of--40 dB or better under themost severe operating conditions--in particular, under directmodulation.

As reported by Haus and Shank in IEEE Journal of Quantum Electronics,Vol. QE12, No. 9, pp 532-539, 1976, the mode spectrum of the originalDFB laser as analyzed by Kogelnik and Shank (J. Appl. Phys., Vol. 43, pp2327-2335, 1972) consisted of modes of equal threshold on either side ofa gap at a `centre` frequency. Haus and Shank note that this thresholddegeneracy is a disadvantage in practical applications where single-modeoperation at a predictable frequency is desired, and they show thatantisymmetric tapering of the coupling coefficient of the period of thestructure may be utilized to remove the threshold degeneracy. Haus andShank established that all structures with an antisymmetric taper of K(the feedback parameter of Kogelnik and Shank) support a mode at thecentre frequency of the local stopbands. This mode has a particularlylow threshold when used in a laser copy.

Haus and Shank found that DFB lasers with a stepped-K structure, that isone with a phase shift between a first section of grating and a secondsection of grating, had no threshold degeneracy and had much betterthreshold discrimination between the fundamental mode and the firsthigher order mode. They also found the frequency separation between thedominant mode and the first order mode to be much greater for thestepped structure than for the uniform structure.

As a result of the work by Haus and Shank, DFB lasers are now made withphase-shifted gratings.

SUMMARY OF THE INVENTION

Ideally, the lengths of the first and second sections are each equal tohalf the total grating length.

While the use of phase-shifted gratings is distinctly beneficial, thereis still a demand for DFB lasers with improved threshold discriminationbetween the fundamental mode and the first higher order mode.Accordingly, the present invention seeks to provide DFB lasers having ahigh level of threshold discrimination between the fundamental mode andthe first higher order mode.

According to a first aspect of the present invention there is provided adistributed feedback laser wherein the means which provide the feedbacknecessary for laser operation are distributed throughout a major portionof the length of the laser, the level of feedback provided by said meansvarying throughout said major portion, characterised in that thefeedback provided by said means at at least one of the ends of saidmajor portion is minimised.

We have discovered that if, instead of the feedback being constantthroughout the length of the means for providing the distributedfeedback, it is modified by a function such that it is a maximum at ornear the middle of the means and tends to zero at one or both of theends of the means, improved mode discrimination can be achieved.

Preferably the function is such that there are no abrupt changes infeedback amplitude throughout the length of the means.

Preferably the means comprises a grating. Preferably the DFB laser isphase-adjusted. Preferably the phase adjustment is provided by means ofa phase-shift in the means. Preferably the phase-shift occurs at or nearthe mid point along the length of the means.

According to a second aspect of the present invention, there is provideda distributed feedback laser wherein means are distributed throughout amajor portion of the length of the laser to give optical feedback, theamount of feedback provided by said means varying throughout said majorportion, characterised in that the amount of feedback varies withoutabrupt changes, the degree of feedback provided by said means tending tozero adjacent at least one of the ends of said portion.

According to a third aspect of the present invention, there is provideda distributed feedback laser wherein a grating is provided to giveoptical feedback, the optical coupling of the grating varying withposition along its length, characterised in that throughout the lengthof the grating there are no abrupt changes in the degree of coupling,and in that adjacent at least one of the ends of the grating thecoupling tends smoothly to zero.

A laser according to an embodiment of the present invention might havean operating wavelength of between 1.3 and 1.55 μm.

Following the work of Haus and Shank described above, others haveconsidered how, other than by building a phase shift into the grating,phase-adjustment could be implemented in semiconductor DFB lasers.Sekartedjo, Broberg, Koyama, Furuya and Suematsu in Jap. J. Appl. Phys.,Vol. 23, No. 10, 1984, pp 791-794, describe a phase-adjusted distributedreflector laser in which phase adjustment is accomplished by providing atrench 0.15 μm deep and 12 μm long across the width of the laser'sgrooved substrate (the device being approximately 800 μm long). Usingliquid phase epitaxy (LPE) a buffer layer nominally 0.15 μm thick wasgrown over the grating, filling the trench (the LPE growth rate beinghigher in the trench). The device's active layer was then grown on thebuffer layer using LPE, the remaining processing steps beingconventional. The optical propagation constant over the trench isdifferent to that in the rest of the waveguide, thereby producing aphase shift in the propagating wave.

Soda, Wakao, Sudo, Tanahashi and Imai have proposed, Electronics Lett.Vol. 20, No. 24, pp 1016-1018, a phase-adjusted DFB laser in whichphase-adjustment is achieved as the result of differences in propagationconstant between a 60 μm long phase-adjustment region which is wider ornarrower than the bulk `uniform` regions at either end of the laser. Thedifference in width between the propagation region and the `uniform`regions is achieved by varying the stripe width. The corrugations of thegrating are formed throughout the length of the device (400 μm) withouta phase shift.

In none of the papers referenced above is there any suggestion thatmodifying the coupling of the grating in such a way that it tends tozero is in any way advantageous.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings inwhich:

FIGS. 1 to 5 are a series of sections in the same plane showing thestages in the production of a DFB laser of known type;

FIG. 6 is a partially exploded perspective view of a conventional ridgewaveguide DFB semiconductor laser;

FIG. 7 is a similar perspective view of a DFB ridge waveguide laseraccording to the present invention, illustrating the new gratingarrangement;

FIG. 8a is a schematic plan view of the grating of the laser shown inFIG. 7;

FIG. 8b is a plot of tooth density against position for the grating ofFIG. 8a;

FIG. 9 is a similar schematic perspective view of a phase-shifted laseraccording to the present invention;

FIG. 9a is an enlarged view of a detail of the laser of FIG. 9;

FIG. 10 is a schematic view of a grating for use in a laser according tothe invention, the grating providing non-uniform coupling as the resultof variations in tooth depth;

FIGS. 11a and 11b are top and side views of an alternative grating foruse in a laser according to the invention, the grating providingnon-uniform coupling as the result of variations in tooth density;

FIG. 12 is a plot of effective e-beam exposure does received by DFBgrating lines when a parabolic weighting function is used withoutcorrection for proximity effects;

FIG. 13 is a similar plot showing the effect of correcting for proximityeffects;

FIG. 14 shows gain difference measured as a function of KL forconventional PSDFB lasers and those according to the present invention;and

FIG. 15 shows yields of devices satisfying a given Δα.L requirement forconventional PSDFB and those according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following description, and elsewhere in this specification, termssuch as "on top of" and "underside" are used. These terms are used forconvenience only and should not be taken to denote a particularorientation of any device unless it is clear from the context that aparticular orientation is intended.

In FIG. 6 an example of a semiconductor DFB laser of conventional type,specifically a DFB ridge-waveguide laser as described in our Europeanpatent application 85301599.8, is shown, and it is in relation to thistype of laser that the invention will be particularly described. Itshould be understood, however, that the present invention is equallyapplicable to other laser structures, including glass lasers and dyelasers, and particularly to other semiconductor laser structures, forexample buried-heterostructure lasers and multiple quantum well lasers.

The first stages in the production of the device, as far as the sectionshown in FIG. 1, are as follows. Onto the (100) face (2 in FIG. 1) of aheavily S-doped InP (n⁺ -type) substrate 1 approximately 200 μm thickwere grown by liquid phase epitaxy (LPE) a series of three layers ofquaternary materials 3, 4, and 5 each 0.2 μm thick. Layer 3 is Te-doped(n-type) of nominal composition Ga₀.17 In₀.83 As₀.36 P₀.64 of band gapequivalent 1.15 um as determined by photoluminescence. Layer 4 isundoped material of nominal composition Ga₀.39 In₀.61 As₀.88 P₀.12 ofnominal band gap equivalent 1.52 um. Layer 5 is similar to layer 3except that it is Zn-doped (p-type). Layer 4, it will be appreciated, isthe active in the finished device and layers 3 and 5 are the lower andupper confinement (or "buffer") layers.

Next, layer 5 was corrugated by chemical etching through anelectron-beam-exposed resist mask in the manner described by Westbrooket al, Electronics Letters, 1982, volume 18, pages 863-865. Thedistributed feedback corrugations 6 are second-order of nominal period0.46 μm, running in the 110 direction, the etching being self-limitingand resulting in triangular grooves with (111)A side walls. The groovesare approximately 0.16 μm deep. The self-limiting nature of the etchingprocess makes for reproducibility and control of the laser feedbackstrength.

Then corrugated layer 5 was overgrown with a layer 7 Zn-doped (p-type)indium phosphide by atmospheric pressure metal organic chemical vapourdeposition (MOCVD) while maintaining the integrity of the gratings aspreviously described (European patent application 84.300240.3 and alsoNelson et al, Electronics Letters, 1983, volume 19, pages 34 to 36). Toachieve this, trimethylindium, triethylphosphine, dimethylzinc,phosphine, and hydrogen were passed over the sample at 100° C. and thesample was heated rapidly to 650° C. whereupon growth occurred. Layer 7was approximately 1.5 um thick.

Then, also by MOCVD, a layer 8 approximately 0.1 um thick of heavilyZn-doped (p⁺ -type) ternary material was grown. The material had thenominal composition In₀.53 Ga₀.47 As.

To complete the structure of FIG. 1, a layer 9 of silica, 0.2 um thick,was grown on top of layer 8 by chemical vapour deposition from silaneand oxygen.

Then the substrate was thinned to 100 um by chemical etching, and theback contact of the laser (i.e. the contact on the underside of thinnedlayer 1) was made by evaporation of tin and gold and subsequentalloying.

The immediately subsequent treatment of the upper layers, as far as FIG.2, was as follows. 0.1 um of titanium (layer 10) and 0.1 um of gold(layer 11) were evaporated onto the silica layer 9. Then about 1 um of apositive resist Kodak 820 was applied to the gold and a dark field maskat right angles to the grating was used to make a stripe window 13between areas of resist 12 and 12'. Windows of 2 um, 4 um, 6 um, and 15um were made on a single wafer.

The steps as far as FIG. 3 were as follows. The structure was exposed toan solution of potassium iodide (4 g) and iodine (1 g) in 40 ml of waterat 20° C. for 1 to 11/2 minutes (which etchant attacks gold layer 11)and to "Countdown silicon dioxide etch (10:1)" for 2 to 21/2 minutes at20° C. (which etchant attacks titanium layer 10 and silica layer 9). Theresult was an undercut etching down to the top layer 8 of semiconductormaterial. By exposing this sequentially to filaments of evaporatingtitanium and gold, an image 14, 15 of the window was obtained on theexposed semiconductor. At the same time, titanium layers 16, 16' andgold layers 17, 17' were deposited on top of the resist 12, 12'. Thewire filaments used were 10 cm away from the target and the thickness ofeach metal deposited was about 0.1 um.

Then the structure shown in FIG. 3 was soaked in acetone for two minutesso as to remove the resist 12, 12' and therewith layers 16, 16', 17, and17'. The result as shown in FIG. 4 is an "initial semiconductorstructure" as referred to with respect to the second aspect of theinvention carrying two sequential layers of metal comprising sub-layers14 and 15 and separate layers of dielectric 9 and 9', which latter arethemselves in this case overlaid with metal 10 and 11 and 10' and 11'.The distance between the edges of the metal layers 14, 15 and the edgeof the dielectric 9, 9' was approximately 4 um.

By treating this structure with 16 weight per cent aqueous H10₃ at 20°C. for 20 seconds to 1 minute (to attack ternary layer 8) and then witha 1:1 mixture by volume of concentrated hydrochloric acid and 90 percent orthophosphoric acid at 20° C. for 30 to 40 seconds (to attackingquaternary layer 5), the result shown in FIG. 5 is received.

In FIG. 5, the etching through layers 8 and 7 has been shown asvertical, although in practice this is unlikely to be the case. It willbe seen that, in the terminology previously used, the base semiconductorportion is constituted by layers 1, 3, 4, and 5; the first elevatedsemiconductor portion (constituting the ridge of the laser) by 7" and8"; the second elevated semiconductor portions by 7 and 8; and the thirdby 7' and 8'. It will be noted that while the first such portion carriesmetal layers titanium 14 and gold 15 in electrical contact therewith theother portions carry the dielectric silica 9 and 9'. The insides ofchannels 16, 17 are substantially free of dielectric and metal.

As is conventional, all the steps above were in fact performed on awafer which was then cut up to yield several devices, a single suchdevice being shown schematically in FIG. 6. In FIG. 6, the second andthird elevated semiconductor portions are omitted so as to show clearlythe relative orientation of the ridge and the grating. The referencenumerals up to 15 have the same significance as previously. Facet 18 ofthe device is a cleaved facet, the other three side facets such as 19being treated (scribed or AR coated) so as to suppress Fabry-Perot lasermodes other than the mode selected by the DFB grating.

FIG. 7 shows schematically one embodiment of the present invention. Asis clear from a comparison of FIGS. 6 and 7, in this embodiment of theinvention, the tooth density of the grating varies along the grating'slength. By reference to FIGS. 8a and 8b, it can be seen that the toothdensity varies parabolically, with tooth spacing increasing towards eachend of the grating. It should be noted that the tooth spacing is not infact a continuous variable; the spacing between adjacent teeth centresis an integer times the fundamental Bragg period.

FIG. 9, a buried heterostructure laser according to the invention isshown with a grating similar to that shown in FIG. 8 apart from theincorporation of a phase shift at its mid-point.

The end facets of the devices shown in FIGS. 8 and 9 are antireflectioncoated. In general it is preferable to have as low a reflectivity aspossible, because the benefit of devices according to the invention isincreased as facet reflectivity is reduced, at least down to 0.1percent. Preferably, therefore, facet reflectivities of 1 percent orless are used. More preferably, the facet reflectivities are less thanor equal to 0.5 percent. Still more preferably, the facet reflectivitiesare 0.1 percent or less. All reflectivities in this specification aredetermined at an operating wavelength of the device, as is conventional.

As an alternative to varying tooth spacing, grating coupling may bevaried according to the invention by varying tooth depth. A schematicexample of this approach is shown in FIG. 10. If electron-beamlithography is used in the grating definition process, a grating inwhich tooth depth varies, as shown in FIG. 10, may be produced byvarying the e-beam exposure dose. A schematic sectional view of thealternative structure, in which coupling is varied by varying toothseparation also shown in FIG. 8, is shown in FIG. 11.

Using electron-beam lithography, this form of grating is produced with auniform e-beam dose with an appropriate exposure pattern (in effect,grooves/teeth are left out with increasing frequency as the grating endsare approached). The proximity effects which are experienced inelectron-beam lithography mean that using the same e-beam dose inwriting each line results in lower exposure for lines which are widelyspaced than for those which are more closely spaced. FIG. 12 illustratesthe effective exposure dose received by DFB grating lines when aparabolic weighting function is used and no correction is made forproximity effects. The lowest exposure is received by lines towards theend of the grating which receive only 73 percent of the peak exposuredose. Such an approach results in faint, uneven or missing gratinglines. This problem can be overcome by using as near as possible auniform e-beam dose in writing the grating. FIG. 13 illustrates theeffective exposure dose received by a DFB grating (with the sameparabolic weighting function as that used for FIG. 12) when proximityeffect correction is used. By increasing the writing time for lines atthe end of the grating the worst case (minimum) exposure has beenincreased to 94.4 percent of the peak exposure dose, resulting in a muchmore uniform etching of grating lines. Since this approach is very easyto implement it is the preferred approach.

It is of course possible to vary grating coupling by combining toothdepth and density variations.

Grating coupling may be varied in a yet further way, by directly varyingthe refractive index of the layer (waveguide) in which the grating isformed. The refractive index may for example be varied by means of alocal implant of diffusion of some index-controlling material. With suchan approach, a conventional grating structure could be used.

The present invention is based on our appreciation that if the level offeedback is gradually reduced to zero at the ends of a laser grating,the side modes are suppressed, increasing the theoretical gaindifference, MSR and single mode device yield (that is the percentage ofdevices from a given wafer which are in fact useably single moded). Ourcalculations have also shown that by combining this weighting with acentral phase shift, a much larger mode suppression ratio can beachieved than is possible using the phase shift alone. FIG. 14 shows thegain difference Δα.L between the principal lasing mode and the strongestside-mode calculated as a function of KL for phase shifted structures ofthis type, together with similar calculations for a conventional phaseshifted DFB laser, assuming zero facet reflectivity. As can be seen fromthis figure, Δα.L for these non-uniform gratings is more than twice thatfor the simple phase-shifted DFB (PSDFB) at KL=2, and continues to riseat larger KL. This increase in Δα.L leads to a substantially largersingle-mode suppression ratio. FIG. 15 shows the percentage yield ofdevices satisfying a given Δα.L requirement, again calculated forweighted and uniform PSDFB structures, assuming a residual facetreflectivity of 0.3 percent (a typical facet reflectivity for AR coatedfacets of modest quality) and a random distribution of facet phases. Asthose skilled in the art will appreciate, a low but non-zero residualfacet reflectivity will tend to improve the single-modedness of uniformPSDFB devices, but will tend to reduce the benefits of the presentinvention relative to the performance of equivalent devices having zerofacet reflectivities. Despite this, it is clear from this graph thatnon-uniform gratings of this type (according to the present invention)produce much higher yields of devices with large Δα.L than areachievable with uniform PSDFB structures.

So far the invention has been described with reference to symmetricalgrating structures, in which coupling is reduced equally towards eachend of the grating. There are, however, applications where it would beadvantageous if the grating had an assymmetrical structure. Inparticular there would be advantages in having grating coupling tendingto zero at only one end of the grating.

As has already been mentioned, the present invention may be realised insome form other than a semiconductor laser, such as, for example, a dyelaser or a doped-glass laser.

For the avoidance of doubt, it should be noted that the termsdistributed feedback laser and DFB laser used in this specification arenot to be taken as referring to distributed bragg reflector (DBR)lasers.

In DBR lasers the grating region is spaced, in the direction ofpropagation (z), from the active region. In effect the grating orgratings function as wavelength-selective mirrors spaced, in thedirection of propagation, from the active region. Conversely, in DFBlasers the grating, while generally spaced in the X or Y direction fromthe active layer, lies parallel to the active layer throughout theentire grating length (but not necessarily throughout the entire activelayer length).

We claim:
 1. A distributed feedback laser comprising a laser medium andoptical feedback to said laser medium, the optical feedback comprisingperiodic perturbations of the refractive index and/or gain of the lasermedium, wherein said periodic perturbations are provided in a gratingregion which is distributed along the laser, the level of feedbackprovided by said perturbations varying throughout said grating region,characterised in that said level of feedback tends to zero at at leastone of the ends of said grating region.
 2. A laser as claimed in claim 1wherein in use, substantially all the optical feedback to said lasermedium is provided by said perturbations.
 3. A laser as claimed in claim1 or claim 2, wherein the amount of feedback provided throughout saidgrating region varies without abrupt changes.
 4. A laser as claimed inclaim 1 wherein said perturbations are provided by a grating havingtooth depth and spacing, and said variation in the level of feedbackprovided by said perturbations is the result of the tooth depth of saidgrating varying along the length of the grating.
 5. A laser as claimedin claim 1, wherein said perturbations are provided by a grating havingtooth depth and spacing, and said variation in the level of feedbackprovided by said perturbations is the result of the tooth spacing ofsaid grating varying along the length of the grating.
 6. A laser asclaimed in claim 4, wherein said variation in the level of feedbackprovided by said perturbations is the result of both the tooth depth andthe tooth spacing of said grating varying along the length of thegrating.
 7. A laser as claimed in any one of claims 4, 5 or 6 whereinsaid grating region extends along substantially the entire length of thelaser.
 8. A laser as claimed in claim 1, wherein said laser comprises asemiconductor injection laser.
 9. A laser as claimed in claim 8 havingat least one optical facet which is antireflection coated and which hasa reflectivity at an operating wavelength of said laser which is onepercent or less.
 10. A laser as claimed in claim 8, said lasercomprising an active layer and a waveguiding layer, wherein the amountof feedback provided throughout said grating region varies withoutabrupt changes and wherein said perturbations are provided by a gratingformed in one surface of said waveguiding layer, variations in thethickness and/or width of the waveguiding layer resulting in saidfeedback variation.
 11. A laser as claimed in claim 8 wherein said lasercomprises a ridge-waveguide structure.
 12. A laser as claimed in claim 8wherein said laser comprises a buried-heterostructure.
 13. A laser asclaimed in claim 8 comprising an indium phosphide substrate.
 14. A laseras claimed in claim 8, said laser having an operating wavelength ofbetween 1.3 and 1.55 μm.
 15. A distributed feedback laser comprising alaser medium and optical feedback to said laser medium, wherein agrating provides said optical feedback, the optical coupling of saidgrating varying with position along its length, characterised in thatthroughout the length of said grating there are no abrupt changes in thedegree of coupling, and in that adjacent at least one of the ends ofsaid grating the coupling tends smoothly to zero.